This invention relates to a method of manufacturing a dry powder particle by generating a charged aerosol stream comprised of charged initially liquid particles and converting said stream into powder particles.
Such a method is well known in the state of the art. By passing a liquid, comprising a solute dissolved in a solvent, through a narrow orifice using high voltage, a charged aerosol stream is generated comprised of charged liquid particles. Evaporation of the solvent results in a fine powder with a relatively narrow size distribution. This, process, known as electrohydrodynamic spraying, is for example suitable for the manufacture of polymeric powders used for electrostatic spraying during powder coating.
The charged powders manufactured according to the state of the art tend to form agglomerates, making them less suitable in the manufacture of high quality finished surfaces. The charged powders also tend to clog up pipelines, which interferes with both production of the powder and use thereof. Thus, the method according to the invention provides a powder having an improved industrial applicability while in addition a higher powder yield is obtained.
In the field of fuel injection it is known to study the phenomenon of mixing two aerosol fuel streams by contacting two similarly charged aerosol fuel streams. (Dunn P. F. et al. the mixing of electrically-charged droplets between and within electrohydrodynamic fine sprays, J. Aerosol Sci. vol. 25, 0.6, pp. 1213-1227, 1994).
It is also known to manufacture a homogeneous mixture by contacting charged granules with an oppositely charged powder (Thesis of P. Vercoulen; Electrostatic processing of particles. Technical University of Delft, the Netherlands). To this end a powder, comprising particles in the micron range, is sprayed and subsequently charged, for example using a corona discharge device, before being contacted with oppositely charged solid granules having diameters in the order of 2 mm. The particulate starting materials were manufactured using conventional techniques, i.e. grinding.
The objects of the present invention are to expand the application possibilities of said method, and in particular to provide a method allowing for the manufacture of powders which could not be produced so far, and to improve the quality of powders produced using said method.
To this end the method according to the invention is characterized in that the charged aerosol stream comprised of charged particles is contacted with a second aerosol stream comprising oppositely charged particles resulting into a combined aerosol stream to form the powder particle.
Electrohydrodynamic spraying allows for the generation of charged aerosol streams having well defined particle size distributions, wherein the liquid particles are charged at the instant the liquid particle is formed, i.e. the droplets do not have to be charged afterwards in a separate charging step.
Thus the present invention provides a method for the manufacture of powder particles, which together constitute a powder, which powder is comprised of less charged or substantially neutral particles. The method allows for, for example, the use of aerosol streams of different composition enabling physical and chemical reactions to occur, and for the coating of particles.
According to a preferred embodiment of the invention the amount of charge of the first aerosol stream and the amount of charge of the second aerosol stream are controlled to yield a substantially neutral powder.
Thus a higher yield of powder can be obtained than previously achievable.
According to a preferred embodiment the particles of at least the first aerosol stream comprising charged particles are generated by electrohydrodynamic spraying.
According to a preferred embodiment the first aerosol stream comprises particles comprising a solvent and a solute, said solvent being evaporated to yield a substantially dry powder particle.
This allows for the manufacture of a powder particles comprising two or more attached subunits. This method is characterized in that a part of the solvent is evaporated from the first aerosol stream to form a first aerosol stream being composed of solid, sticky charged particles which is contacted with the second charged aerosol stream comprising solid charged particles resulting in the combined aerosol stream which is converted into an essentially dry powder. Thus it is possible to make aerosol particles with a specific shape, the subunits having the same or a different composition.
A particularly preferred embodiment of the method according to the invention is characterized in that, when contacted, both the first and the second aerosol stream comprise liquid charged particles.
This embodiment allows for a multitude of physical and chemical reactions to occur.
Advantageously the method according to the invention comprises, after contacting the first aerosol stream with the second aerosol stream, separation of particles with the desired composition according to their charge to mass ratio.
As the amount of charge is proportional to the mass of the particle (or, in case of a particle comprising an evaporating solvent, to the mass of the solute), the composition of the particle is reflected in the charge to mass ratio of the particle formed.
Yet another preferred method of manufacturing a powder according to the invention is characterized in that at least the first aerosol stream, being comprised of larger particles and satellite particles, is subjected to a particle separation step providing a substantially monodisperse aerosol stream whereafter the substantially monodisperse aerosol stream is contacted with the oppositely charged aerosol stream.
This embodiment, which is especially useful with electrohydrodynamic spraying, provides for a powder to be produced having a even narrower particle size distribution. Accordingly it is preferred that both the first and the second aerosol stream are subjected to the particle separation step.
According to an advantageous embodiment, the separation step is conducted using a grounded electrode. The grounded electrode, placed near or in that part of the aerosol stream where the particles to be removed pass and more distantly spaced from the desired particles, serves as a simple and effective means to attract and remove the unwanted particles. Thus an aerosol stream enriched in the desired particles is obtained and subsequently brought into contact with the oppositely charged aerosol stream.
The invention also relates to powder comprising ceramic precursor powder particles, polymer-comprising powder particles or powder particles comprising a pharmaceutical compound, for example coated powder particles, as well as a pharmaceutical composition comprising such a pharmaceutical compound-comprising powder together with a pharmaceutically acceptable carrier or diluent.
Moreover the invention relates to a method of manufacturing a ceramic product characterized in that a ceramic precursor powder according to the invention is sintered.
The method allows for the manufacture of very fine ceramic precursor powders without grinding as well as powders with compositions which until now could not be obtained or only with great difficulty. If the above mentioned separation step is performed, providing two aerosol streams comprised of satellite droplets, highly desired ceramic precursor nanoparticles can be obtained, suitable for defect free ceramic products.
Finally, the invention relates to an apparatus for working the method according to the invention, comprising an electrode having an inlet and an outlet, a channel connecting the inlet and the outlet, the outlet being an orifice in the centre of an electrically conducting area opening into a chamber, wherein the electrically conducting area is defined by a barrier, said barrier preventing the flow of liquid beyond the electrically conducting area and the chamber having a counter electrode as well as an exhaust opening for a product formed by electro-hydrodynamic spraying.
The apparatus is characterized in that the chamber is a mixing chamber and the apparatus is provided with a second electrode having an orifice opening into the chamber.